California Geologic History

Antler and Sonoma Events Around 400 million years ago the quiescent period inwestern North America came to an end.Antler Orogeny-Development of an offshore island arc. Theevidence for this event is strongest farther east, but there areindications in the northern Sierra Nevada and Klamathregions. Possibly similar to Japan today.Sonoma Orogeny-250 mya-This event is most evident incentral Nevada, which may have been the location of thecoast at the time. The orogeny seems to have marked thejoining of the island arc containing rocks now found in theSierra Nevada foothills and central Klamath Mountains withthe rest of the continent. This event appears to mark the firsttime the coast moved west into most of California.